It's more of a side effect that the class can define these properties of the same name. The real goal is that in the following example, the private property #x is not affected.

  class A {
    #x = 100;
  }

  const a = new A();
  a.x = 200;

On top of this, you should be able to define private variables and methods in derived classes without interfering with the base class private properties:

  class A {
    #x = 100;

    getA() { return this.#x }
  }

  class B extends A {
    #x = 200;

    getB() { return this.#x }
  }

  const b = new B();
  b.getA() // 100
  b.getB() // 200;